The present invention relates to a hydraulic brake system comprising a pedal-operated brake booster, which is connected with the master cylinder and which has an auxiliary pressure supply system, a booster piston, and a booster chamber wherein an auxiliary pressure is established which is proportional to the foot pressure. The hydraulic brake system is provided with wheel brakes connected to the master cylinder and with a filling stage cylinder with a two-stage piston displaceably housed therein. A pressure chamber is provided in front of the large stage of the two-stage piston and a filling chamber is arranged in front of the small stage of the two-stage piston. The filling chamber communicates with one of the working chambers of the master cylinder and/or a brake line by way of a pressure line.
A device featuring the above characteristics is known from German printed patent application (DE-OS) No. 31 08 908.9. In the known brake booster, a master cylinder piston is arranged in a first cylinder bore, forming one component together with a booster piston. In this arrangement, a tappet connects the master cylinder piston and booster piston with each other. A pedal-side front face of the booster piston confines a booster chamber formed within the housing and is arranged coaxially in respect of a pedal-operable piston rod. Hinged to the piston rod, on its part, is a lever arrangement for the actuation of a brake valve. By applying a force to the piston rod and by actuating the brake valve, it is possible to supply a pressure into the booster chamber which is proportional to the actuating force and which displaces the booster piston and the master cylinder piston connected with the booster piston in the direction of actuation of the power booster. Accordingly, a corresponding pressure is built up in the working chamber of the master cylinder. Connected to the working chamber of the master cylinder are wheel brakes which are pressurized in this manner. Accordingly, a corresponding slowing-down of a vehicle results. Upon pressurization of the booster chamber, the piston rod connected with the brake pedal will likewise be pressurized. Thus, a corresponding reaction force will be felt at the pedal which will provide feed-back to the drive of the vehicle with regard to the intensity of the braking operation initiated by him.
Vehicles with a high permissible total weight need a high boosting factor so as to keep the foot pressure and the pedal travel within the desired limits. Upon a failure of the booster, this will lead to undesirable or even unduly high foot pressures and short pedal travels. It is thus an object of the present invention to provide a brake system of the type described which in the even of a booster failure, works with a reduced effective master brake cylinder surface and which can be coupled with brake slip monitoring and controlling electronics.